sha2: provide support for `spki::AlgorithmIdentifier` #304

Job	Run time
minimal-versions / minimal-versions	26s
set-msrv / set-msrv	3s
test (1.71.0)	28s
test (stable)	29s
build (1.71.0, thumbv7em-none-eabi)	19s
build (1.71.0, wasm32-unknown-unknown)	20s
build (stable, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	17s
	2m 39s

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